Conceptual Design and Analysis of a Natural Gas Assisted Coal-to

Developing coal-to-olefins processes is therefore of great interest to many ... However, people have to face and pay for the severe environmental ...
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Conceptual Design and Analysis of a Natural Gas Assisted Coal-toOlefins Process for CO2 Reuse Siyu Yang,† Qingchun Yang,† Yi Man, Dong Xiang, and Yu Qian* School of Chemical Engineering, South China University of Technology, Guangzhou 510641, China S Supporting Information *

ABSTRACT: Olefins have been regarded as one of the most important platform chemicals. The production of olefins and their derivatives are highly subject to oil. Developing coal-to-olefins processes is therefore of great interest to many countries, especially China. However, people have to face and pay for the severe environmental problems resulting from coal-to-olefins development. Of these problems, high CO2 emission of a coal-to-olefins process, always attracts the most attention since it is about five to six times of that of an oil-to-olefins process. For this problem, this paper proposes a new natural gas assisted coal-toolefins process integrating CO2 recovery gasification and CH4/CO2 reforming techniques. The former technique increases the amount of syngas from the gasifier, while the latter one uses additional natural gas reacting with CO2 to produce H2-rich syngas. Key parameters are studied during the simulation of the new process. The advantages of the process are manifested by comparison with a conventional coal-to-olefins process from the techno-economic point of view. Results show that the new process is promising since it reduces the CO2 emission by 29.9% and increases the carbon efficiency and the energy efficiency by 20.7% and 7.8%. With the high market price of natural gas, the product cost of the new process is slightly higher than the coal-toolefins process. But the new process will be more competitive if considering that the carbon tax is larger than $18.2/t CO2 or that shale gas is available in China. huge exergy loss. Ren7 made the comparative study of energy utilization and CO2 emission of different processes that produce value-added FT synthetic oil and olefins from different energy resources, such as oil, natural gas, and biomass. From the aspect of CO2 emission, the oil-to-olefins process emits the least amount of CO2, followed by the gas-to-olefins process. The coal-to-olefins process is the dirtiest one emitting five to six times of that of the oil-to-olefins process. Moreover, the high CO2 emission results in serious loss of carbon element. Thus, the coal-to-olefins process also faces the lower resource utilization efficiency than the oil-to-olefins process. There are some other studies making economical comparison of coal-toolefins and oil-to-olefins processes. Xiang8 did a comparative analysis of a 1500 kt/y naphtha-to-olefins process and a 600 kt/ y coal-to-olefins process. An interesting conclusion was made that the product cost of the coal-to-olefins process is 72% of the oil-to-olefins process. If introducing a carbon tax equal to or larger than $60/t CO2, the cost advantage of coal-to-olefins will no longer be present.9 In a word, coal-to-olefins processes are fruitfully developed in China because of the high reserve of coal and the low product cost. However, development of coal-toolefins processes is seriously criticized because of the accompanying high emissions. It is therefore an urgent need to solve the problem of high CO2 emission. Until now, there have been several technologies for CO2 mitigation. They are increasing resource and energy efficiencies of the process itself, exploiting renewable energy,

1. INTRODUCTION Olefins are one of the most important platform chemicals used in production of large numbers of derivative chemicals. However, olefins production currently faces the severe dilemma that increasing depletion of oil worsens the conflict between the supply and demand of olefins. This problem is even worse in China where the self-sufficiency rates of ethylene and propylene are only 50% and 64%.1 Many countries endeavor to explore other carbon-rich resources for chemical production,2 such as coal, natural gas, coke oven gas, and biomass, with increasing acknowledgement of the high market price and large market fluctuation of olefins.3 ExxonMobil started the methanol-toolefins project from the early 1970s as well as the projects of biomass and natural gas to olefins. The Lugri and Statoil companies had already succeeded in commercialization of methanol-to-olefins projects.4 China is making a great effort to develop alternatives to oil-to-olefins processes. The key technical breakthrough is the successful commercialization of the dimethyl ether and methanol to olefins (DMTO) synthesis technique developed by the Dalian Institute of Chemical Physics. This technique was applied to 1800 kt/y coal-to-olefins project in Baotou, China.5 The main reason to develope coalto-olefins projects is largely because of the special energy composition, in which oil takes 16.2%, coal, 74.7%, and natural gas, 2.7%.6 This coal dominant situation will last for a long time, and the development of coal-to-olefins processes is promising in China. However, coal-to-olefins processes suffer from the problems of higher environmental impact and lower resource and energy efficiency compared to oil-to-olefins processes. Coal gasification is accompanied by large amounts of greenhouse gas (GHG) emissions. The high irreversibility of combustion gives rise to © 2013 American Chemical Society

Received: Revised: Accepted: Published: 14406

June 20, 2013 September 3, 2013 September 9, 2013 September 9, 2013 dx.doi.org/10.1021/ie401937k | Ind. Eng. Chem. Res. 2013, 52, 14406−14414

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Figure 1. Schematic diagram of the conventional CTO process.

Figure 2. Schematic diagram of the NGaCTO process.

2. NOVEL NATURAL GAS ASSISTED COAL-TO-OLEFINS PROCESS A natural gas assisted coal-to-olefins process (NGaCTO) is proposed based on conventional CTO processes. Before introducing NGaCTO, let us make a brief description of a conventional CTO process. A CTO process generally consists of several main unit processes: the air separation unit (ASU), coal gasification unit (CG), acid gas removal unit (AGR), sulfur recovery unit (SR), water gas shift unit (WGS), methanol synthesis unit (MS), and methanol to olefins unit (MTO). The flow sheet of the conventional CTO is shown in Figure 1. The coal-gasified syngas is featured as carbon-rich gas with H2/CO as low as 0.5. This H2/CO is not high enough for methanol synthesis requiring for the H2/CO around 2. Thus, additional H2 is provided by the water gas shift reaction in the conventional CTO. The disadvantage of the shift reaction is that it is accompanied with high emission of CO2, inevitably resulting in decline of resource utilization. Syngas is then cleaned by the AGR unit where H2S is recovered as elemental sulfur and most of CO2 is separated and emitted to the environment. According to the work of Ren,7 there are nearly 5.8 t CO2 emitted for production of 1 t olefins. Thus, it is concluded that the high emission and low resource utilization weaken the competitive strength of CTO processes. To solve the above problems, this paper proposes the NGaCTO process integrating a CH4/CO2 reforming reaction. This reaction, which is also named as dry methane reforming reaction, is regarded as one of the promising CO2 mitigation techniques capable for large-scale industrial processes. Xie13 adopted the CH4/CO2 reforming reaction in the coal-based Fischer−Tropsch (FT) syntheses. Gangadharan14 combined the H2O/CO2 and the CH4/CO2 reforming reactions in the hydrogen production processes. On the other hand, a CH4/ CO2 reforming reaction is strongly endothermic. It is therefore high-energy-consuming to use this reaction for CO2 mitigation. For efficient energy utilization, the NGaCTO recycles part of CO2 back into the gasifier as the gasification agent. Several additional reactions take place in the gasifier to increase the

adopting CO 2 capture and sequestration (CCS), and developing CO2 conversion. Of these techniques, increasing resource or energy efficiency is usually the first step and straight way to reduce CO2 emission. Heat integration and optimization are the most common methods for this increase but are often difficult to implement with limitations from a technical point of view. Replacing coal by renewable energy resources has been reported as the most efficient way. However, some limitations, such as low reserve and high application cost, determine that renewable energy resources still cannot be widely used. As for CCS, additional subprocesses have to be involved, increasing the energy consumption and therefore decreasing the energy efficiency of the process. For example, the Integrated Gasification Combined Cycle (IGCC) with CCS has an energy efficiency about 10% lower than that without CCS.10 On the other hand, CCS techniques do not convert CO2 to chemicals but geographically store it, leaving the buildup of the carbon element unrelieved. Thus, the most sensible way for CO2 mitigation is converting CO2 to other chemicals.11 According to the above discussion, this paper proposes a natural gas assisted coal-to-olefins process aiming at maximal reuse of CO2. This process recycles part of CO2 to the gasifier as a gasification agent to increase the syngas production.12 Another part of the CO2 is collected to react with CH4 in the CH4/CO2 reforming reaction. CO2 is used as feedstock to generate syngas with H2/CO equal to 1. As CO2 is used as a feedstock, this novel process could decrease the CO2 emission and increase the resource utilization efficiency at the same time. Key parameters of the new process are studied during simulation of the process. The advantages of the new process are manifested by comparison with a conventional coal-toolefins process from a techno-economic point of view. In the end, the economic performance of the new process with the carbon tax is also analyzed compared to the coal-to-olefins process. 14407

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Figure 3. Simplified flow diagram of the NGaCTO process.

processes.17 In this paper, we focus on modeling the coal gasification unit and the DMR unit. A simplified flow diagram of the NGaCTO process is shown in Figure 3, and the key operational parameters for the simulation could be found in Table A1 in the Supporting Information. 3.1. Modeling of the Coal Gasification. In modeling of the coal gasification, the Texaco gasifier is selected. Conventional solid and unconventional solid are modeled by the classes without particle size distribution, MIXCINC. The property method uses PENG-ROB. Coal and volatiles are defined as unconventional solids. The decomposition reaction is model by RStoic model. The composition of the reaction products depends on the property of input coal, i.e. analysis basis and dried basis of coal and sulfur content. The process of coal gasification is modeled by the minimum free energy of the RGibbs model. The detailed modeling and simulation refers to our previous work on coal-to-methanol16 and coal-to-olefins processes.17 For efficient energy utilization, a part of CO2 is recycled back into the gasifier as gasification agents. From here on, this gasification with CO2 recycled is called CO2 recovery gasification. However, the gasification performance will deteriorate with excessive CO2 recycling.18 The main reactions in this gasification are shown as follows.

production of syngas. More details of these reactions will be described in the Modeling and Simulation of the NGaCTO section. In this case, there is less energy required for CO2 mitigation. However, with excessive CO2 recycled to the gasifier, the reactivity of CO2 and the gaisifer temperature will be decreased. Even worse, H2/CO of the syngas will decrease at the same time.15 Thus, it is important to determine an appropriate distribution of CO 2 between the CH4/CO2 reforming reaction and the gasifier. There are therefore two key parameters in design of the NGaCTO process, the mass proportion of CO2 for gasification (α) and for CH4/CO2 reforming reaction (β). The proportion of CO2 released to the environment (γ) is equal to one minus the sum of α and β. Thus, the final CO2 emission is changed as α and β vary. In our study, the emission is decreased as α and β increases. With more and more CO2 recycled, however, the conversion rate would be reduced gradually and the decreasing rate of emission is increasingly decayed. That is because that the temperatures of the gasification and the dry reforming reaction is supposed to be isothermal or changed in a fixed small range in our study. The flow sheet of the NGaCTO process is shown in Figure 2. Different from the conventional CTO process, the syngas from the gasifier is mixed with the syngas generated by the dry methane reforming (DMR) unit. Because the H2/CO of the later syngas is equal to 1, the mixed syngas could have H2/CO increased up to near 0.8. The WGS unit is used to further increase the H2/CO to about 2.1. The syngas is then fed into the AGR unit for removal of CO2, H2S, COS, HCN, NH3 with Rectisol method. The CO2 is separated in the CO2 desorber and recycled back to the DMR unit and the gasifier after preheated. Finally a minor of CO2 is emitted to the environment. Most H2S is separated in the H2S desorber, which is then reduced to elemental sulfur in the SR. The clean syngas is then fed sequentially into the methanol synthesizer and the olefins synthesizer. The output mixture is finally separated into different products, such as ethylene, propylene, and C4 and C5 byproducts.

C + CO2 ↔ 2CO,

θ ΔH298 = + 172.5 kJ/mol

CO + H 2O ↔ CO2 + H 2 ,

(1)

θ ΔH298 = − 41.2 kJ/mol

(2)

It can be seen that the above reactions are generally endothermic. This means that recycling CO2 to the gasifier will reduce combustion heat and decrease gasification temperature, giving negative impact on the gasification performance. Thus, it is necessary to employ an appropriate CO2 recovery gasification for the NGaCTO. 3.2. Modeling of the CH4/CO2 Reforming. The CH4/ CO2 reforming reaction is the main reaction in the DMR unit as shown in eq 3. There is also a side reaction which reduces CO2 to CO by H2 as shown in eq 4.

3. MODELING AND SIMULATION OF THE NGACTO It is necessary to model and simulate the NGaCTO process prior to feasibility analysis. The NGaCTO is modeled by Aspen Plus and Peng−Robinson (PR-BM) is selected as the property method. The detailed modeling and simulation refers to our previous work on coal-to-methanol16 and coal-to-olefins

CH4 + CO2 ↔ 2CO + 2H 2 , θ ΔH298 = +247.1 kJ/mol

14408

(3)

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θ ΔH298 = + 41.2 kJ/mol

CO2 + H 2 ↔ CO + H 2O,

the productivity of syngas if the gasification temperature is not decreased much. He also stated that the final amount of CO is increased although some of them are converted into CO2 in the preceding water gas shift reaction. To prove the availability of appropriate CO2 recycled in the NGaCTO, we compare the NGaCTO with the one with only the CH4/CO2 reforming reaction. The two processes are designed to produce the same quantity of olefins. The NGaCTO has α equal to 0.3 and β equal to 0.4, while the other one has β equal to about 0.5. It is to see how many CO2 released and how much energy are consumed in these two processes. Results are shown in Figure 4. It is seen that the CO2 reduction of the process with only

(4)

Ni/Rh/Al2O3 catalyst is used for the CH4/CO2 reforming reaction. According to the property of this catalyst, the reaction temperature should be higher than 600 °C. Olsbye19 reported that the best operational temperature range is between 600 and 1040 °C in order to avoid from coking. In this paper, we fix the operational condition at 800 °C and 0.1 MPa. According to the work of Kugler,20 the conversion rate and the selectivity of CO and H2 will be close to 100% if the space velocity of input stream is between 0.2 and 0.3 m3/(g h).20 The dynamics of the CH4/CO2 reforming reaction makes use of Langmuir− Hinshelwood. The reaction rate equation and its corresponding parameters with a 95% confidence value are formulated as follows:21 rDMR =

k WGSK CH4K CO2(pCH pCO − (pCO2 pH 2 /Keq)) 4

2

2

1 + pH −0.25 pCH K CH4 + K CO2pCO pH 2

4

2

0.25 2

(5)

⎛ 332.04 ± 52.40 ⎞ ⎟ k WGS = 3.59 × 1021exp⎜ ⎝ ⎠ RT

(6)

⎛ −109.68 ± 57.53 ⎞ ⎟ K CH4 = 2.89 × 10−8exp⎜ ⎝ ⎠ RT

(7)

⎛ −125.39 ± 39.11 ⎞ ⎟ K CO2 = 3.53 × 10−8exp⎜ ⎝ ⎠ RT

Figure 4. Difference between the NGaCTOs with/without CO2 recycled.

(8)

where rDMR is the reaction rate of the CH4/CO2 reforming reaction; kWGS denotes the reaction rate constant of the water gas shift reaction; KCH and KCO2 denote the absorption equilibrium constants of CH4 and CO2; Keq is the reaction equilibrium constant of the CH4/CO2 reforming reaction. pCH4, pCO, pCO2, and pH2 are partial pressures of CH4, CO, CO2, and H2, respectively. According to the literature,14 the best fit kinetic parameters for the reaction are shown in Table A2 in the Supporting Information. In the DMR unit, CO2 from the AGR unit is mixed with natural gas and then preheated by the output gas from the DMR unit. The Peng−Robinson method is used for modeling.14 The reactor is modeled by RPlug model and the diameter and the length of the DMR unit are fixed according to the work of Lim22 to ensure the conversion rate larger than or equal to 99.9%. 3.3. Analysis of Key Operational Parameters. According to the above discussion, there are two key operational parameters in the NGaCTO process: the proportions of CO2 recycled back to the gasifier (α) and CO2 for the CH4/CO2 reforming reaction (β). In the following sections, we first discuss the advantage of CO2 recovery by comparing the two processes with/without CO2 recovery. Successively, we study the effect of these two parameters on chemical conversion and energy consumption. Advantage of the Process with CO2 Recovery Gasification. The CH4/CO2 reforming reaction is strongly endothermic. With excessive dry reforming reaction, CO2 reduction rate in the expense of one unit energy is low. In another word, the energy consumption for generating the same magnitude of products are much higher. Li et al.18 reported that directing the appropriate amount of CO2 back to the gasifier could increase

CH4/CO2 reforming reaction is slightly more than the NGaCTO, about 4.85%. However, the energy consumption of NGaCTO is much smaller than the other one by 19.68%. Thus, including CO2 recovery gasification is necessary to reduce the burden of energy consumption for the NGaCTO. Effect of α on the Gasification. Employing appropriate CO2 recovery gasification is necessary for the NGaCTO. It means that the appropriate α need to be determined. For this, we first analyze the mass ratio of CO2 recycled over the input coal, denoted by CO2/coal, and design it increasing from 0.1 to 1. The change of the gasification temperature and the composition of syngas are analyzed and shown in Figure 5. It can be seen from the figure that the amount of CO and CO2 increases but that of H2 decreases slightly with CO2/coal

Figure 5. Effect of CO2/coal on gasification unit. 14409

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from 28.98% up to 55.46%. Beside the above two indexes, we also estimate the effect on the CO2 emission rate, which is defined as the ratio of the amount of C element in CO2 emitted and that in feedstock. As shown in Figure 6, the emission rate decreases obviously down to one-fifth. Thus, we could say that the NGaCTO process could obtain better and better resource utilization efficiency as the proportion of CO2 used for CH4/ CO2 reforming reaction increases. The resource utilization efficiency increases as β increases. However, CH4/CO2 reforming reaction is endothermic. There will be more energy consumption if a larger amount of CO2 were used for this reforming reaction. It is therefore necessary to analyze the effect on energy utilization as well. The energy efficiency of the NGaCTO process (η) is defined as the ratio of product energy Epd and the total input energy Ein,

increases. In the meantime, the gasification temperature decreases quickly. According to the specification of Texaco gasifiers, the best operational temperature range is between 1300 and 1400 °C.23 According to our previous work on a coalto-methanol process,16 to keep the temperature in this range we could infer that α should be smaller than or equal to 0.3. For maximal energy efficiency, we select the largest a equal to 0.3. Effect of β on Energy and Resource Utilization. The proportion of CO2 for the CH4/CO2 reforming reaction (β) is another key operational parameter for the NGaCTO process. As β increases, the amount of CO2 converted is spontaneously increased, but at the same time more and more energy is consumed. Thus, it is important to select an appropriate β for the best trade-off of energy and resource utilization. For analysis, we design the β value increasing from 0 to 0.7 and investigate its effect on the production of olefins, element efficiency, and energy efficiency. Before this investigation, it is necessary to build the analysis models of resource and energy efficiencies. Carbon element efficiency EE(C) is commonly used for analysis of resource utilization of chemical processes based on fossil fuels. In the NGaCTO process, C2H4 and C3H6 are the only products. There are also some other components, such as ash, alkane, and C4 and C5 products, considered as either waste or byproducts. The mass balance is built and shown as eq 9 and EE(C) is formulated as eq 10,

η=

= 2fout (C2H4) + 3fout (C3H6) + fout (CO2 )

EE(C) =

2fout (C2H4) + 3fout (C3H6) fin (coal) + fin (CH4)

E in

× 100%

(11)

where Ein is the total energy input, including feedstock and utilities (steam and electricity). The consumed electricity and steams for the conventional CTO process refers to our previous work.17 The major difference of utility consumption between the NGaCTO and the CTO is caused by newly introducing the DMR unit. The utility needed for this unit consists of steams used for heating the reforming reaction and electricity supplied for compression work. These utilities are calculated according to the simulation results in Aspen Plus. The effect of β on energy efficiency is studied in this paper as α is fixed to 0.3. The energy efficiency of the NGaCTO process is shown in Figure 7. In this study, β is increased from 0 to 0.7.

fin (coal) + fin (CH4)

+ fout (others)

Epd

(9)

× 100% (10)

where f in(coal) and f in(CH4) denote the flow rates of input coal and CH4 streams; fout(C2H4), fout(C3H6), and fout(CO2) are the flow rates of output C2H4, C3H6, and CO2 streams; fout(others) is the sum of the flow rates of streams of other components. The effects of β on the production of olefins, CO2 emission, and carbon element efficiency are shown in Figure 6. It can be seen that the production increases from 29.85 to 103.33 t/h as β increases. The EE(C) of the NGaCTO process increases

Figure 7. Effect of β on energy efficiency and utility energy.

It is seen that the energy consumption increases quickly in the first half stage and become smooth in the second half stage. From this result, we first concluded that β ranging from 0.5 to 0.7 is the appropriate region for this process. On the other side, we study the utility spent as β increases. It is found from the figure that the utility consumption is increased quickly as more and more CO2 joins in the CH4/CO2 reforming reaction. At β equal to 0.7, the utility consumption for the NGaCTO is 6.6 times of that of the conventional CTO process. Thus, in the course of new process design, we set the β equal to 0.5 for energy saving in the following study.

Figure 6. Effect of β on production, EE(C), and CO2 emission rate. 14410

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Table 1. Benchmark Case for Investments16,26

4. COMPARISON BETWEEN NGACTO AND CTO According to the discussion in the third section, we select an appropriate α and β equal to 0.3 and 0.5. To manifest the advantage of the NGaCTO process, we compare it with a conventinal CTO process. This comparison is based on the same production scale of 600 kt/y olefins and from both technical and economic points of view. 4.1. Technical Performanc. For technical comparsion, we compare the energy efficiency, the carbon element efficiency, and the CO2 emission of the two processes. The simulation of the CTO process refers to my previous work.16 The comparison is shown in Figure 8. It can be seen that the

benchmark ASU coal preparation coal gasification WGS

oxygen supply daily coal input daily coal input caloric value

H2S removal

element S

carbon removal SR

CO2 absorption sulfur output syngas input

methanol synthesis dry reforming olefins synthesis

caloric value methanol input

reference scale

scale factor

domestic made factor

reference investment (M$)

21.3 kg/s

0.50

0.50

45.70

27.4 kg/s

0.67

0.65

29.10

39.2 kg/s

0.67

0.80

78.00

1450 MW 29.3 mol/s 327 t/h

0.67

0.65

39.30

0.67

0.65

44.44

0.67

0.65

32.80

29.3 mol/s 1081 mol/s 716 MW

0.67

0.65

22.90

0.67

0.65

20.40

0.67

0.65

61.90

62.5 kg/s

0.67

1

223.06

capital investment.17 After calculation, the total capital investments of the NGaCTO process and the CTO process are $5052 and $4915 1/t. Distributions of the total capital investments of these two processes are shown in Figure 9. Figure 8. Technical comparison of the NGaCTO and the CTO processes.

NGaCTO process has the energy efficiency η and the carbon element efficiency EE(C) higher than those of the CTO process by 7.8% and 20.7%. The CO2 emission ratio, which was defined as the ratio of the amount of carbon element in CO2 and feedstock,26 is lower than that of the CTO process by 29.9%. We therefore could say that the NGaCTO process is more efficient than the CTO process from both resource and energy utilization. 4.2. Economic Performance. Analyzing economic performance is necessary for introducing a new process to be industrialized. We analyze the capital cost and the product cost of the NGaCTO process in the following sections. Analysis of Capital Investment. Total capital investment is one of the important indexes for assessing the economic performance. It consists of the fixed capital and the working capital investments. Capital investment is estimated by the exponential coefficient method as eq 12. ⎛ Q ⎞n I2 = θI1⎜⎜ 2 ⎟⎟ ⎝ Q1 ⎠

Figure 9. Capital cost distribution of the NGaCTO and the CTO processes.

Analysis of Product Cost. As for the product cost of the NGaCTO, we calculate the operating cost and the general cost. The estimation of the product cost is also based on the work of Peters and Orhan;24,25 the cost of the raw materials and utilities are calculated according to the simulation results; the operating labor cost is calculated according to Han’s work.27 Table 2 lists the unit price of raw material and utilities. The total product cost of these two processes are $1132 and $1048/t. The product cost distributions of the two processes are shown in Figure 10.

(12)

where I1 and I2 are the reference and the practical investments; Q1 and Q2 indicate the reference and the practical scales; θ represents the domestic made factor; and exponent n is the production scale factor. In this paper, the determination of the scale factor depends on the methods of increasing production scale. We calculate equipment investment of different units of these two processes. Table 1 lists the investment benchmark of different units. After calculating the equipment investment, the rest part of the fixed-capital investment could be derived based on the work of Peters and Orhan,24,25 while the working capital of the coal to olefins process is usually set to be 17% of the total

Table 2. Unit Prices of Raw Materials and Utilities

14411

items

price

coal ($/t) methane ($/m3) water ($/m3) heating cost ($/GJ) electricity ($/kWh) CO2 compression ($/t)

100 0.4 0.35 5.98 0.11 10.5

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Figure 10. Product cost distribution of the NGaCTO and the CTO processes.

Figure 11. Effect of carbon tax on product cost.

conventional CTO as the environmental policy is increasingly tightened.

It can be seen from the figure that both the capital investment and the product cost of the NGaCTO process are slightly higher than the CTO plant. This is not strange because those additional capitals are expended by introducing the DMR unit. At the same time, the introduction of the DMR unit will increase the cost of utilities and maintenance. On the other hand, the product cost of the NGaCTO process is also higher than that of the CTO process. This is mainly because that the market price of natural gas is high in China. This high price is determined by the severe conflict between demand and supply resulting from the short reserve of natural gas. In this case, the government tends to explore alternatives of natural gas. Shale gas is now regarded as the major potential alternative for natural gas, especially after it was found that China has the largest reserve and the U.S. made a big breakthrough in mining and exploitation technologies.28 In the U.S., it is reportedly possible to extract shale gas on a commercial scale an data reasonable cost.29,30 However, this technology has not been passed to China with different causes. China is still suffering the difficulty of developing self-research mining technology. However, the research of shale gas mining has been already written in the government’s 12th Five-Year Plan. With the increasing acknowledgement of environmental impact, especially GWP, governments of different countries are paying more and more attention to GHG emissions. For GHGs mitigation, a carbon tax policy has been applied which will become increasingly tighter in large numbers of countries.31 In the academic field, it is increasingly accepted to involve the carbon tax in techno-economic analyses of chemical and energy processes.32 Thus, we put the carbon tax in comparison to the NGaCTO and the CTO processes, finding its effect on economic performance. We design the carbon tax increase from $0 to $60/t CO2. The change of the total product cost is shown in Figure 11. It can be seen that both the product cost of the two processes increases as the tax increases. However, the CO2 emission for the NGaCTO process is only 1.2 t/t olefins much smaller than that of the CTO 4.8 t/t olefins. Thus, the product cost of the CTO process increases much quicker than that of the NGaCTO process. The competitive carbon tax is $18.2/t CO2 for the NGaCTO. If the tax is smaller than this value, the product cost of the CTO process is lower than or equal to that of the NGaCTO process. In contrast, the NGaCTO will be better than the CTO process in terms of the product cost, if the tax is larger than this value. This value is much smaller compared to the carbon tax of Norway released in 2008. This means that the NGaCTO process is superior to the

5. CONCLUSIONS AND REMARKS In the momentum of reducing CO2 emission of the coal-based chemical processes, we proposed a novel coal-to-olefins process with hybrid natural gas, NGaCTO process. The natural gas is mainly used in the CH4/CO2 reforming reaction for CO2 mitigation. To approach the trade-off of resource and energy utilization, we drive some part of CO2 recycled back to the gasifer. In this CO2 recovery gasification, CO2 is also converted to CO with nearly no additional energy consumption. For analysis of the new process, we built the mathematical model and simulate it with the analysis of two key parameters. One is the ratio of CO2 recycled to the gasifier and the other is the ratio of CO2 joining the reforming reaction. According to the study, we found that the relation between these two parameters and resource and energy utilization. In the last part of this paper, we analyzed the advantages and disadvantages of the new process compared to the conventional CTO process. From the technical point of view, the NGaCTO process manifest 2 times higher at carbon efficiency, 1.5 times lower at CO2 emission rate, and 1.3 times higher at energy efficiency than those of the CTO process. In contrast, from the economic point of view, the NGaCTO is slightly worse than the CTO process. It has the product cost higher than that of the CTO process by 8%. This is reasonable because the NGaCTO introduces the new reforming unit process and the unit price of the natural gas is high in China. However, the result will be overthrown if considering carbon tax. Finally, the economic comparison with carbon tax is studied in this paper. We found that the NGaCTO will manifest strong competitiveness if the carbon tax is larger than $18.2/t CO2. We also believe that the NGaCTO will manifest the astonishing advantage compared to the conventional CTO process, if the technical bottleneck of shale gas is solved in the next five years.

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ASSOCIATED CONTENT

S Supporting Information *

This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-20-87113046, +86-13802902300. E-mail: [email protected]. Website: http://www2.scut.edu.cn/ce/ pse/qianyuen.htm. 14412

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Author Contributions

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†

S.Y. and Q.Y. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for financial support from the China NSF key project (No. 21136003), the China NSF project (No. 21306056), and the National Basic Research Program (No. 2012CB720504; No. 2014CB744306).

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NOMENCLATURE

Capital Letters

Ej = the energy of flow j EE = element efficiency I = project investment, M$ Ki = absorption equilibrium constant of component i. R = gas constant, kJ/(kmol·K) Q = production capacity, M$

Lowercase Letters

n = scale factor f in (j) = the flow rate of input flow j, kmol/h fout (j) = the flow rate of output flow j, kmol/h i = component in syngas (i = CH4, H2O, CO, CO2) kWGS = reaction rate constant of water gas shift reaction pi = partial pressure of component i in syngas, MPa r = reaction rate, kmol/(kg cat·s) yi = mole fraction of component i in syngas Greek Letters

α = proportion of CO2 recycled for gasification in total CO2 β = proportion of CO2 for reforming reaction in total CO2 γ = proportion of emitted CO2 in total CO2 θ = domestic made factor η = energy efficiency, %

Acronyms

AGR = acid gas removal unit ASU = air separation unit CCS = carbon dioxide capture and sequence CG = coal gasification CTO = coal-to-olefins DMR = dry methane reforming DMTO = dimethyl ether and methanol to olefins HTS = high-temperature shift LTS = low-temperature shift MS = methanol synthesis unit MTO = methanol-to-olefins NGaCTO = natural gas assisted coal-to-olefins SR = sulfur recovery unit WGS = water gas shift unit

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REFERENCES

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